1. Field of Invention
The present invention generally relates to a system and method for unraveling biological curvilinear/tubular structures utilizing curved cross-sections.
2. Description of Prior Art
Disorders of the GI tract are common, including neoplasms, inflammatory diseases, post-operative complications, malabsorption, infectious and ischemic diseases. Colorectal carcinoma is the third cause of cancer death in the United States, common in other Western countries and increasing rapidly in the East where Western life-styles are being adopted. It was estimated that 109,000 new diagnoses of colon cancer and 43,000 of rectal cancer were made in 1993, divided almost equally between males and females. In the same year approximately 50,000 patients died of colon cancer and 7,000 of rectal cancer. Inflammatory bowel diseases affect nearly two million Americans, and almost 10,000 new cases each of ulcerative colitis and Crohn's disease are diagnosed annually in the United States alone. Although inflammatory bowel diseases are more frequent in developed areas of the world, there are no racial boundaries limiting their incidence.
Fiberoptic colonoscopy currently represents the most sensitive imaging examination of the colonic mucosa to detect lesions. However, colonoscopy is invasive, quite expensive, carries a small risk of perforation, involves sedation, and in some cases fails to visualize the entire colon. With the advent of spiral computed tomography CT, CT colography (CTC) has attracted increasing attention as an alternative to the traditional colonoscopy for 3D visualization of the colon.
GI tract diagnosis has relied on X-ray imaging since early this century, and has been a major impetus for the development of better technology and methods. GI imaging has undergone several revolutionary changes in the past two decades, especially X-ray CT, magnetic resonance imaging (MRI), endoscopy, and endosonography.
X-ray CT is an established cost-effective imaging modality that produces sectional and volumetric maps of linear X-ray attenuation coefficient. Spiral (or helical) CT is a major advance in X-ray CT in which source rotation and patient translation are simultaneously and continuously conducted, resulting in a helical scanning locus with respect to a patient.
The advantages of spiral CT are volumetric scanning within a single breath-hold and retrospective position selection for transaxial slice reconstruction. Fast scanning eliminates slice-to-slice misregistration, since projection data are rapidly acquired at a consistent inspiration level. Typically, a 30 cm long volume can be covered in 15 to 30 seconds. Retrospective reconstruction allows overlapped transaxial slices and thus provides improved longitudinal resolution as compared to conventional incremental CT (step and shoot). In conventional CT, there are fewer reconstructed slices available, and features with sizes comparable to the slice thickness may be missed or distorted. Spiral CT decreases the radiation hazard since the maximum amount of radiation dose delivered to individual voxels is minimized. Recognized for these advantages, spiral CT has gained general acceptance as the standard medical CT mode. Currently spiral CT abdominal scans are routinely done nationwide.
Spiral CT allows coverage of the entire colon within a breath-hold acquisition. From this volumetric data set, 3D volume rendering algorithms can be employed to perform the "virtual" endoscopic visualization of the mucosal surface of the colon for detection of colorectal polyps, precursors of cancer. These techniques require navigation through the tortuous anatomy of the colon, which can be time consuming in practice.
Conceptually, the current CTC may be viewed as a two-step process: navigation and visualization. In other words, a curvilinear colon path must be first determined either interactively or automatically, then the colonic mucosa is visualized after either surface or volume rendering. This method produces excellent internal views in a cine loop, but it can be difficult to use and time consuming, since the colon is highly convoluted.
In G. Wang, M. W. Vannier, "Unraveling the GI tract by spiral CT," SPIE 2434, pp. 307-315, 1995, a shift from slice-based GI tract examination to unraveled GI tract examination was proposed. At that time, GI tract examination with CT and MRI was commonly interpreted by slice-based visual inspection despite the volumetric nature of the anatomical components, tumors/lesions and imaging modalities.
The slice-based interpretation primarily has two limitations. First, global comprehension of GI components and tumors/lesions are often difficult; and, second, quantification is impossible due to subjective interpretive error sources. The proposed spiral CT abdominal image visualization and analysis system utilizing an unraveling system based upon use of planar cross-sections orthogonal to the central path of the curvilinear/tubular structure was better suited to handle the highly convoluted GI tract. The system straightened and flattened convoluted portions of the GI tract via computer post-processing of the CT data. Using such a system has potential to improve diagnostic performance by providing intuitive and quantitative pathologic information.
An unraveling approach allows the manipulation the GI tract virtually and the derivation of pathological information quantitatively and automatically. An advantage of this approach is the direct visualization of the colon surface, without the need for navigation. Also, the image unraveling utilities can be combined with virtual reality techniques and conventional orthogonal sections for greater visualization freedom.
However, the progress on unraveling of the GI tract has been limited by substantial artifacts from unraveling with planar cross-sections orthogonal to the central path. Specifically, a fundamental difficulty with unraveling convoluted structures such as the colon is that resampling along intersecting image planes can result in discontinuities, misregistration, multiple counting or total missing of polyps.
When planar cross-sections with respect to the colon path are stacked, the tortuous colon is straightened, which is referred to as "hard" straightening. Hard straightening is satisfactory in regions where the colon is linear, but stacking transverse sections of a curved lumen can produce double sampling of some regions and inadequate sampling of other regions, especially at sharp turns or kinks in the lumen. FIG. 1 demonstrates these problems. In FIG. 1A, a single polyp 110 along the inner curve can appear in several sections 120, be missed in subsequent sections 130 and be sampled again in later sections 140. In this way as seen in FIG. 1B, one polyp 150 appears as two polyps, "double counting" occurs. For the same reasons, a polyp 160 along the outer portion of a curve may not be sampled and will be missed entirely.
"Soft" straightening/unraveling techniques were developed to overcome the limitations of hard straightening. In soft straightening, cross-sections with respect to the colon path are curved appropriately so that these slices do not conflict with each other.